First off, your Nair ***** his Pikmin throw, so although youll have to eventually approach, you have that tool you can use so you can think it out. Its important to realize what Pikmin is next, if its not blue or white, I believe you CAN outrange the grab (sadly Im not to sure, I know we can **** Purple), but your timings gotta be perfect. The best approach I think is to get behind him from a whiffed grab, either using a Nair to try and get him airborne for just a second, or air dodging around to just get behind him, than pressure him, his grab has no super armor, so if you keep the jab strings up, he will eventually crack, unless he has really good timing. Once he's airborne, our aerials IMO, **** his, as long as you space well, Id use that to further keep him airborne and unsafe. Anything to keep Olimar staggered and/or off the ground, your jabs can wreck him, even if he jumps out, which isnt to hard albeit, he has a huge weakness below him, simply exploit that into a well read Upsmash, a good side Fair, or continue jabbing when hes near the ground. Ike can be easily gimped, but I believe Olimar cannot do that at all, where as you have that power if your smart.
They key is simply to be aware of what he has at the moment, and keep him off the ground.